The control of molecular weight increase (which is directly related to intrinsic viscosity) during a solid state polycondensation process is important because variations of reaction vessel residence time, temperature, reaction products partial pressure, and gas oxygen content affect the quality of the final polymer. Also, an uncontrolled temperature profile within the reaction vessel or reactor can result in the formation of blocks of polymer material created by superficial melting and adhesion of polymer chips, a phenomena that leads to process shutdown.
Prior patents (Rothe et al, U.S. Pat. Nos. 4,064,112 and Herron, 4,161,578) disclose a fixed bed reactor for solid state polycondensation of polyethylene terephthalate chips. These references, however, do not advocate continuous monitoring of intrinsic viscosity and control of process parameters in response to the monitored values. Such monitoring may be desirable since variation from a specific intrinsic viscosity can result in the production of polymer chips which fail to meet specifications and which do not have the final desired polymer quality or character.
Though the variation of polycondensation rate is attributed primarily to a variation in chip size, initial polymer history, residence time, reactor geometry, temperature distribution, and partial pressure of ethylene glycol in the reactor gas, in a fixed bed reactor, the primary means to vary the polycondensation rate is by varying the residence time in the reactor or by changing the gas flow rates and temperature in the reactor.
Previously it has been customary to obtain knowledge of the polycondensation rate by laboratory testing. However, difficulty is encountered using laboratory test procedures because of the very lengthy total time required to complete the tests. A conventional system may require of the order of from six to twelve hours, for example, between changing a process parameter and receiving the intrinsic viscosity test results. Also, even after testing it is difficult to determine whether chips will agglomerate and stick together thereby shutting down the reactor. The present invention seeks to overcome these difficulties in a continuous bed reactor.